One package, stable, moisture curable, alkoxy-terminated organopolysiloxane compositions

ABSTRACT

Stable, substantially acid-free, one package moisture curable alkoxy-terminated organopolysiloxane compositions are provided having a condensation catalyst and a stabilizer or scavenger for materials having chemically combined hydroxy radicals in the form of an N-silyl-substituted imidazole.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to copending application of John E. Hallgren, Ser. No. 277,525, now U.S. Pat. No. 4,377,706, for Polyalkoxysilylenolethers and method for making, White et al, Ser. No. 277,524, now U.S. Pat. No. 4,395,526, for One Package, Stable, Moisture Curable, Polyalkoxy-Terminated Organopolysiloxane Compositions and Method for Making, filed concurrently on June 26, 1981, and John J. Dziark, for Scavengers for One Component Alkoxy Functional RTV Compositions and Process, Ser. No. 349,695, filed Feb. 17, 1982, now U.S. Pat. No. 4,417,042 and copending application Ser. No. 476,000, filed Mar. 17, 1983, Swiger et al, for Scavengers for One Package RTV Compositions. Reference is also made to the copending application of Mary Ann White et al, Ser. No. 481,524 and my copending applications Ser. No. 481,527, Ser. No. 481,528, Ser. No. 481,529, for One Package, Stable, Moisture Curable, Alkoxy-Terminated Organopolysiloxane Compositions, and Ser. No. 481,526 for a Method for Making an Enoxy Stabilized Room Temperature Vulcanizable Organopolysiloxane Composition Which Resists Color Change Upon Aging, where all of these applications are filed concurrently herewith, assigned to the same assignee as the present invention and incorporated herein by reference.

BACKGROUND OF THE INVENTION

Prior to the present invention, various one and two-package moisture curable room temperature vulcanizable (RTV) compositions were available based on the use of a silanol-terminated polydiorganosiloxane having the formula, ##STR1## where R is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical, which is preferably methyl, or a mixture of a major amount of methyl and a minor amount of phenyl, cyanoethyl, trifluoropropyl, vinyl, and mixtures thereof and n is an integer having a value of from about 50 to about 2500, with a cross-linking silane having hydrolyzable radicals attached to silicon.

Ceyzeriat, U.S. Pat. No. 3,133,891 and Bruner, U.S. Pat. No. 3,035,016, are based on the use of methyltriacetoxysilane with a silanol-terminated polydimethylsiloxane under substantially anhydrous conditions. Although the one-package compositions of Bruner or Ceyzeriat, upon exposure to atmospheric moisture, provide satisfactory one-package room temperature vulcanizable organopolysiloxane compositions exhibiting satisfactory tack-free time, for example, 30 minutes or less after an extended shelf period, the acetic acid by-product is corrosive and has a disagreeable odor.

Other variations of one-package acyloxy acid generating RTV's are shown by Kulpa, U.S. Pat. No. 3,296,161, Goossens, U.S. Pat. No. 3,296,195 and Beers, U.S. Pat. No. 3,438,930, assigned to the same assignee as the present invention. Additional one-package acyloxy acid generating RTV compositions are shown by Schulz et al, U.S. Pat. No. 3,647,917 and Nitzsche et al U.S. Pat. No. 3,886,118.

An improved, low odor, substantially non-corrosive one-package RTV composition is shown by Beers, U.S. Pat. No. 4,257,932, assigned to the same assignee as the present invention. Beers achieves a reduction in odor and corrosive properties by utilizing as a cross-linking silane, a less volatile material such as methyltris-(2-ethylhexanoxy)silane.

A non-corrosive two package moisture curable organopolysiloxane composition free of carboxylic acid generating groups is shown by Nitzsche et al, U.S. Pat. No. 3,127,363 which is based on the use of a polyalkoxysilane, or polysilicate cross linking agent, in place of methyltriacetoxysilane. The ingredients of the two package noncorrosive composition of Nitzsche et al, are mixed under atmospheric conditions and the resulting composition must be used soon after the ingredients are mixed because the resulting blend has a short shelf life. Although the mixture of Nitzsche et al, which is typically polyalkoxysilane, silanol-terminated polydiorganosiloxane and tin soap catalyst, provides upon mixing, a fast curing non-corrosive room temperature vulcanizable composition, the Nitzsche et al mixture does not have the extended shelf life advantage of the one package system which is required for various commercial uses and therefore is excluded from a variety of applications.

Nitzsche et al, U.S. Pat. No. 3,065,194, teaches that a mixture of an endblocked dimethylsiloxane polymer, such as hydroxy and alkoxy endblocked, inert filler, ethylorthosilicate and dibutyltindilaurate can be vulcanized upon contact with water, after a 14 day shelf period at room temperature. However, the various ingredients of the mixture have to be vigorously dried by heating for 1 hour at 200° C., and the RTV, after a relatively short shelf period, has to be drenched with water.

Improved results toward combining the advantages of a non-corrosive acid-free polyalkoxysilane cross-linking agent with a silanol-terminated polydiorganosiloxane as a one-package system are shown by Weyenberg, U.S. Pat. No. 3,334,067, Cooper et al, U.S. Pat. No. 3,542,901 and by Smith et al U.S. Pat. Nos. 3,689,454, and 3,779,986, the last two being assigned to the same assignee as the present invention, utilizing a titanium chelate catalyst in place of a tin catalyst. However, after room temperature vulcanizable one-package systems based on a titanium chelate catalyst were allowed to age for a period of 5 hours or more, it was found that the tack-free time of the aged RTV was considerably longer than the tack-free time of the same mixture after it was initially mixed and immediately exposed to atmospheric moisture.

As shown by Brown et al U.S. Pat. No. 3,122,522, a platinum catalyst is used to prepare an alkoxy terminated silalkylenepolysiloxane polymer. However, this method of synthesizing the base polymer requires an expensive hydrosilylation procedure. Additional efforts to achieve a desirable non-corrosive, substantially odor-free stable one-package RTV based on the use of polyalkoxy organo polysiloxane in a more economic manner are shown by Brown et al, U.S. Pat. No. 3,161,614 or U.S. Pat. No. Re. 29,760. Brown et al employed a polyalkoxy end blocked polysiloxane which was based on the use of a mineral acid generating polyalkoxyhalosilane, and a curing catalyst. However, these compositions were found to be unusable because they failed to cure in contact with a tin catalyst, even in the presence of moisture.

As utilized hereinafter, the term "stable" as applied to the one package alkoxy-terminated organopolysiloxane RTV's of the present invention means a moisture curable mixture capable of remaining substantially unchanged while excluded from atmospheric moisture and which cures to a tack-free elastomer after an extended shelf period. In addition, a stable RTV also means that the tack-free time exhibited by freshly mixed RTV ingredients under atmospheric conditions will be substantially the same as that exhibited by the same mixture of ingredients exposed to atmospheric moisture after having been held in a moisture resistant and moisture-free container for an extended shelf period at ambient conditions, or an equivalent period based on accelerated aging at an elevated temperature.

Valuable, substantially acid-free cured products have been obtained with the use of particular organosilane scavengers, as taught in the afore-described application of White et al, Ser. No. 277,524 now U.S. Pat. No. 4,395,526, for One Package, Stable, Moisture Curable, Polyalkoxy-Terminated Organopolysiloxane Compositions and Method for Making. Likewise, as shown in White et al, copending application Ser. No. 481,524, filed concurrently herewith, silicon-free organic scavengers have been found to provide substantially acid-free cured products from RTV compositions which in many instances are more economical than those shown in Ser. No. 277,524 now U.S. Pat. No. 4,395,526.

It has been found that although these organosilanes or silicon-free organic materials can serve as scavengers for materials having chemically combined hydroxy radicals in the RTV compositions, an amine accelerator is generally required in combination with such scavengers to achieve a stabilized RTV composition which has a satisfactory cure time.

The present invention is based on the discovery that stable, substantially acid-free, one package moisture curable RTV compositions can be made by utilizing as the scavenger for chemically combined hydroxy radicals, an N-silyl-substituted imidazole of the formula, ##STR2## which has been found to be effective as both a stabilizer and cure accelerator of substantially acid-free, fast curing, polyalkoxy-terminated organopolysiloxane compositions, where R¹ is selected from a C.sub.(1-13) monovalent hydrocarbon radicals and substituted C.sub.(1-13) monovalent hydrocarbon radicals, R², R³ and R₄ are selected from hydrogen, C.sub.(1-8) alkyl, C.sub.(1-8) alkenyl and C.sub.(6-13) aryl and a is an integer equal to 1-4 inclusive.

Accordingly, the above-described N-silyl-substituted imidazoles of formula (2) can be advantageously used in combination with alkoxy-terminated polydiorganosiloxane of the formula ##STR3## where R and n are as previously defined, R⁵ is a C.sub.(1-8) aliphatic organic radical selected from alkyl radicals, alkylether radicals, alkylester radicals, alkylketone radicals and alkylcyano or a C.sub.(7-13) aralkyl radical, R⁶ is a C.sub.(1-13) monovalent organic radical selected from R radicals as previously defined, and b is a whole number equal to 0 to 2 inclusive. It has been found that there can be utilized from about 0.1 to about 10 parts of the N-silyl substituted imidazole of the present invention per 100 parts of the alkoxy-terminated polydiorganosiloxane of formula (3) or the silanol terminated polydiorganosiloxane of formula (1). Preferably, 1 or 2 parts of the N-silyl substituted imidazole per 100 parts of organopolysiloxane polymer can be used.

Although the N-silyl imidazoles of formula (2) can function as both a stabilizer and curing accelerator for the room temperature vulcanizable alkoxy-terminated organopolysiloxane compositions of the present invention, in particular instances curing accelerators such as certain organic amines can be used in combination with the N-silyl imidazoles to further enhance the cure rate of the resulting RTV composition. Furthermore, there can be utilized in the room temperature vulcanizable compositions of the present invention cross-linking polyalkoxysilane having the formula, ##STR4## where R⁵ and R⁶ are as previously defined and c is a whole number equal to 0 or 1.

STATEMENT OF THE INVENTION

There is provided by the present invention, a one-package, substantially anhydrous, room temperature vulcanizable organopolysiloxane composition stable under ambient conditions over an extended period of time in the substantial absence of moisture and convertible to a substantially acid-free, tack-free elastomer comprising by weight,

(A) an organopolysiloxane wherein the silicon atom at each polymer chain end is terminated with at least one alkoxy radical,

(B) an effective amount of condensation catalyst

(C) from 0 to 0.1 part, per part of the alkoxy terminated organopolysiloxane of (A) of a cross-linking polyalkoxysilane of formula (4)

(D) 0 to 0.1 part of an amine accelerator, per part of the alkoxy terminated organopolysiloxane and

(E) a stabilizing amount of a N-silyl-substituted imidazole of formula (2).

Radicals included within R of formulas (1) and (3) are, for example, aryl radicals and halogenated aryl radicals, such as phenyl, tolyl, chlorophenyl, naphthyl; aliphatic and cycloaliphatic radicals, for example, cyclohexyl, cyclobutyl; alkyl and alkenyl radicals, such as methyl, ethyl, propyl, chloropropyl, vinyl, allyl, trifluoropropyl; and cyanoalkyl radicals, for example, cyanoethyl, cyanopropyl, cyanobutyl.

Radicals included within R¹ of formula (2) are, for example, C.sub.(1-8) aliphatic radicals, such as C.sub.(1-8) alkyl, for example, methyl, ethyl, propyl, butyl, pentyl, etc.; alkenyl radicals, such as vinyl, allyl; aralkyl radicals such as benzyl, phenethyl; C.sub.(6-13) aromatic radicals, for example, phenyl, tolyl, xylyl, naphthyl; halogenaated C.sub.(1-8) aliphatic and halogenated C.sub.(6-13) aromatic. Radicals included within R⁵ are, for example, alkylether radicals such as 2-methoxyethyl; alkylester radicals, for example 2-acetoxyethyl; alkylketone radicals, for example 1-butan-3-onyl; alkylcyano radicals, for example 2-cyanoethyl; in addition, R¹ also can be selected from R⁵ O-- radicals, where R⁵ is as previously defined. Radicals included by R², R³ and R⁴ are further selected from R radicals as previously defined. Radicals included within R⁶ also are selected from R radicals as previously defined. In formulas (1-4), where R, R¹, R², R³, R⁴, R⁵ and R⁶ can be more than 1 radical, these radicals can be the same or different.

The N-silyl imidazoles of formula (2) which can be utilized in the practice of the present invention as a stabilizer curing accelerator for the room temperature vulcanizable compositions of the present invention can be made by a procedure shown in L. Birkoffer and A. Ritter, Angew. Chem., Int. Ed., Eng. 4, 427 (1965).

Some of the silyl imidazoles which are included within formula (2) are, for example,

N-trimethylsilylbenzimidazole;

N-trimethylsilyl-2-methylimidazole;

N-trimethylsilyl-4-methylimidazole;

N-trimethylsilyl-5-methylimidazole;

N-trimethylsilyl-4,5-diphenylimidazole;

methyltriimidazoyl silane;

The alkoxy-terminated organopolysiloxanes which are included within formula (3) can be made by various procedures. One procedure is taught by Cooper et al U.S. Pat. No. 3,542,901 involving the use of a polyalkoxysilane with a silanol-terminated polydiorganosiloxane in the presence of an amine catalyst. For example, a cross-linking silane of formula (4) can be used in accordance with Cooper et al's method to end-cap a silanol-terminated polydiorganosiloxane of formula (4).

In instances where a silanol terminated polydiorganosiloxane of formulas (1) is used directly in making RTV compositions in accordance with the practice of the present invention, there can be used from 0.001 to 0.1 part of the cross-linking silane of formula (4) per part of the silanol polydiorganosiloxane.

Some of the cross-linking polyalkoxysilanes included within formula (4) are, for example, methyltrimethoxysilane; methyltriethoxysilane; ethyltrimethoxysilane; tetraethoxysilane; vinyltrimethoxysilane; etc.

Included among the amine accelerators which can be used in combination with the silyl imidazole of formula (2) to accelerate the cure of the RTV composition of the present invention are for example, silyl-substituted guanidines and alkyl substituted guanidines. Some of the silyl-substituted guanidines are shown by Takego U.S. Pat. Nos. 4,180,642 and 4,248,993.

In addition to the above described amine accelerators, which can be used with the silyl imidazoles, there also are included for example, compounds containing amidine, amine oxide, aminopyridine, azo, azomethine, azoxy, cyanate, hydrazide, hydrazine, hydrazone, imine, isocyanate, isothiocyanate, oxazone, nitroso, thiocyanate, thiourea, urea, and urethane functionalities and nitrogen containing heterocyclics such as imidazoles, imidazolines, isoxazoles, lactams, oxazoles, purines, pyrazines, pyrazoles, pyrazolines, pyrimidines, pyrroles and thiazoles.

In addition to the above substituted guanidines, there can be used various amines, for example, di-n-hexylamine, dicyclohexylamine, di-n-octylamine, hexamethoxymethylmelamine, and silylated amines, for example, γ-aminopropyltrimethoxysilane and methyldimethoxy-di-n-hexylaminosilane. Methyldimethoxy-di-n-hexylaminosilane acts as both a scavenger and curing accelerator. The primary amines, secondary amines, silylated secondary amines are preferred, and secondary amines, and silylated secondary amines are particularly preferred. Silylated secondary amines such as alkyldialkoxy-n-dialkylaminosilanes and guanidines such as alkyldialkoxyalkylguanidylsilanes which are useful as cure accelerators herein also act as scavengers and, in certain instances, as stabilizers in the compositions of this invention.

As previously described the alkoxy terminated organopolysiloxane of formula (3) can be made by using a polyalkoxysilane with a silanol-terminated polydiorganosiloxane. The silanol-terminated polydiorganosiloxanes which can be used to make the polyalkoxyorganopolysiloxanes are well known and preferably have a viscosity in the range of from about 100 to about 400,000 centipoise and more preferred from about 1000 to about 250,000 centipoise when measured at about 25° C. These silanol-terminated fluids can be made by treating a higher molecular weight organopolysiloxane, such as dimethylpolysiloxane with water in the presence of a mineral acid, or base catalyst, to tailor the viscosity of the polymer to the desired range. Methods for making such higher molecular weight organopolysiloxane utilized in the production of silanol-terminated polydiorganosiloxane of formula (1), are well known. For example, hydrolysis of a diorganohalosilane such as dimethyldichlorosilane, diphenyldichlorosilane, methylvinyldichlorosilane, or mixtures thereof, can provide for the production of low molecular weight hydrolyzate. Equilibration thereafter can provide for higher molecular weight organopolysiloxane. Equilibration of cyclopolysiloxane such as octamethylcyclotetrasiloxane, octaphenylcyclotetrasiloxane, or mixtures thereof, will also provide for higher molecular weight polymers. Preferably, such polymers are decatalyzed of equilibration catalyst by standard procedures prior to use, such as shown by Boot U.S. Pat. No. 3,153,007, assigned to the same assignee as the present invention.

Silanol-terminated organopolysiloxanes having viscosities below 1200 centipoises can be made by treating organopolysiloxanes consisting essentially of chemically combined diorganosiloxy units with steam under pressure. Other methods that can be employed to make silanol-terminated polydiorganosiloxanes are more particularly described in U.S. Pat. No. 2,607,792 to Warrick and U.K. Pat. No. 835,790.

Effective amounts of the condensation catalysts which can be used in the practice of the present invention to facilitate the cure of the RTV compositions are, for example, 0.001 to 5 part based on the weight of 100 parts of the silanol-terminated polydiorganosiloxane of formual (1). There are included tin compounds, for example, dibutyltindilaurate; dibutyltindiacetate; dibutyltindimethoxide; carbomethoxyphenyl tin tris-uberate; tin octoate; isobutyl tin triceroate; dimethyl tin dibutyrate; dimethyl tin di-neodeconoate; triethyl tin tartrate; dibutyl tin dibenzoate; tin oleate; tin naphthenate; butyltintri-2-ethylhexoate; tinbutyrate. The preferred condensation catalysts are tin compounds and dibutyltindiacetate is particularly preferred.

Titanium compounds which can be used are, for example, 1,3-propanedioxytitanium bis(ethylacetoacetate); 1,3-propanedioxytitanium bis(acetylacetonate); diisopropoxytitanium bis(acetylacetonate); titanium naphthenate; tetrabutyltitanate; tetra-2-ethylhexyltitanate; tetraphenyltitanate; tetraoctadecyltitanate; ethyltriethanolaminetitanate. In addition beta-dicarbonyltitanium compounds as shown by Weyenberg U.S. Pat. No. 3,334,067 can be used as condensation catalysts in the present invention.

Zirconium compounds, for example, zirconium octoate, also can be used.

Further examples of metal condensation catalysts are, for example, lead 2-ethyloctoate; iron 2-ethylhexoate; cobalt 2-ethylhexoate; manganese 2-ethylhexoate; zinc 2-ethylhexoate; antimony octoate; bismuth naphthenate; zinc naphthenate; zinc stearate.

Examples of nonmetal condensation catalysts are hexylammonium acetate and benzyltrimethylammonium acetate.

Various fillers and pigments can be incorporated in the silanol or alkoxy-terminated organopolysiloxane, such as for example, titanium dioxide, zirconium silicate, silica aerogel, iron oxide, diatomaceous earth, fumed silica, carbon black, precipitated silica, glass fibers, polyvinyl chloride, ground quartz, calcium carbonate etc. The amounts of filler used can obviously be varied within wide limits in accordance with the intended use. For example, in some sealant applications, the curable compositions of the present invention can be used free of filler. In other applications, such as the employment of the curable compositions for making binding material on a weight basis, as much as 700 parts or more of filler, per 100 parts of organopolysiloxane can be employed. In such applications, the filler can consist of a major amount of extending materials, such as ground quartz, polyvinylchloride, or mixtures thereof, preferably having an average particle size in the range of from about 1 to 10 microns.

The compositions of the present invention also can be employed as construction sealants and caulking compounds. The exact amount of filler, therefore, will depend upon such factors as the application for which the organopolysiloxane composition is intended, the type of filler utilized (that is, the density of the filler and its particle size). Preferably, a proportion of from 10 to 300 parts of filler, which can include up to about 35 parts of a reinforcing filler, such as fumed silica filler, per 100 parts of silanol-terminated organopolysiloxane is utilized.

In the practice of the present invention, the room temperature vulcanizable compositions can be made by agitating, for example stirring, a mixture of materials which can consist of a polyalkoxy terminated diorganopolysiloxane, a silyl imidazole, cross-linking silane of formula (4) which can be optional, and amine accelerator which also can be optional, where the blending is performed in the substantial absence of atmospheric moisture. Preferably, the condensation catalyst can be added also in the substantial absence of atmospheric moisture.

As used hereinafter, the expressions "moisture-free conditions" and "substantially anhydrous conditions", with reference to making the RTV compositions of the present invention, mean mixing in a dry box, or in a closed container which has been subjected to vacuum or nitrogen purge to remove air, which thereafter is replaced with a dry inert gas, such as nitrogen. Temperatures can vary from about 0° C. to about 180° C. depending upon the degree of blending, the type and amount of filler.

A preferred procedure for making the RTV compositions of the present invention is to agitate under substantially anhydrous conditions, a mixture of the polyalkoxy terminated organopolysiloxane, filler, along with an amount of polyalkoxysilane of formula (4) sufficient to effect the substantial elimination of hydroxy functional groups in the mixture. This "end-capping" procedure can require several minutes, hours, or even days, depending upon such factors as the amount of silanol on the filler, the nature of the --OR⁵ group on the cross-linking silane, etc. There then can be added to the substantially silanol-free mixture the condensation catalyst, cross-linking silane or mixture thereof, along with other ingredients, for example, the curing accelerator and pigments.

In order that those skilled in the art will be better able to practice the invention, the following examples are given by way of illustration and not by way of limitation. All parts are by weight.

EXAMPLE 1

There was prepared by initially mixing under substantially anhydrous conditions 100 parts of a methyldimethoxy end-capped polydimethylsiloxane having a viscosity of about 15-30×10³ centipoise at 25° C. and 0.2 part of dibutyltindiacetate. There was then added to the resulting mixture, 2 parts of N-(trimethylsilyl)imidazole (TMSI) which was obtained from the Aldrich Chemical Company and used without additional purification. The resulting room temperature vulcanizable composition was heat aged at 95°±5° C. in an oil bath under substantially anhydrous conditions. A similar room temperature vulcanizable composition free of TMSI was also heat aged following the same procedure. Tack-free times (TFT) were then determined after exposing 1-2 mm thick samples of the respective compositions to a 58% humidity atmosphere at room temperature. The following results were obtained where TFT is in minutes:

                  TABLE I                                                          ______________________________________                                         TFT After Heat                                                                 Aging (95 ± 5° C.)                                                   TMSI    0 hr        24 hr    48 hr                                             ______________________________________                                         None    180 min.    >18 hr   no cure (>70 hr)                                  2 parts 100 min     110 min  120 min                                           ______________________________________                                          The above results show that the TMSI is an effective stabilizer and      accelerator for the room temperature vulcanizable alkoxy-terminated      organopolysiloxane compositions of the present invention.

EXAMPLE 2

The procedure of Example 1 was repeated, except there was utilized a methyldimethoxy end-capped polydimethylsiloxane having a viscosity of about 35×10³ at 25° C. In addition, there was utilized in combination with TMSI, 0.6 part of di-n-hexylamine per 100 parts of the methyldimethoxy end-capped polydimethylsiloxane. The procedure of Example 1 was repeated to determine the TFT in minutes after heat aging at 95°±5° C. The following results were obtained:

                  TABLE II                                                         ______________________________________                                         TMSI    TFT (min) after Heat Aging (95 ± 5° C.)                      (Parts) 0 hr       24 hr    48 hr                                              ______________________________________                                         0        40        45       Sample gelled                                      1       <14        19       34                                                 1.5     <14        19       34                                                 2.0     <14        19       34                                                 ______________________________________                                    

The above results show that the TMSI can perform as both a curing accelerator and stabilizer for the room temperature vulcanizable polyalkoxy terminated organopolysiloxane compositions of the present invention. In addition, the rate of cure can be enhanced by addition of di-n-hexylamine curing accelerator.

Although the above examples are directed to only a few of the very many variables involved in the practice of the present invention, it should be understood that the present invention is directed to a much broader variety of room temperature vulcanizable organopolysiloxane compositions utilizing N-silyl-substituted imidazoles of formula (2) in effective amounts with polyalkoxy terminated organopolysiloxanes of formula (3) and optionally polyalkoxysilanes of formula (4) along with an amine accelerator as shown in the description preceding these examples if desired. 

What I claim as new and desire to secure by Letters Patent of the United States is:
 1. A one-package, substantially anhydrous, room temperature vulcanizable organopolysiloxane composition stable under ambient conditions in the substantial absence of moisture over an extended period of time and convertible to a substantially acid-free, tack-free elastomer comprising by weight,(A) an organopolysiloxane wherein the silicon atom at each polymer chain end is terminated with at least one alkoxy radical, (B) an effective amount of condensation catalyst (C) from 0 to 0.1 part, per part of the alkoxy terminated organopolysiloxane of (A) of a cross-linking polyalkoxysilane, (D) 0 to 0.1 part of an amine accelerator, per part of the alkoxy terminated organopolysiloxane and (E) a stabilizing amount of a N-silyl-substituted imidazole of the formula ##STR5## where R¹ is selected from C.sub.(1-8) aliphatic radicals, substituted C.sub.(1-8) aliphatic radicals, C.sub.(6-13) aromatic hydrocarbon radicals and substituted C.sub.(6-13) aromatic hydrocarbon radicals, and R², R³ and R⁴ are selected from hydrogen, C.sub.(1-8) alkyl, C.sub.(1-8) alkenyl, and C.sub.(6-13) aryl, and a is an integer equal to 1-4 inclusive.
 2. A room temperature vulcanizable composition in accordance with claim 1, where the alkoxy-terminated polydiorganosiloxane is a methyldimethoxy terminated polydmethylsiloxane.
 3. A room temperature vulcanizable composition in accordance with claim 1, where the condensation catalyst is a tin compound.
 4. A room temperature vulcanizable composition in accordance with claim 1, where the N-silyl-substituted imidazole is N-(trimethylsilyl)imidazole.
 5. A room temperature vulcanizable composition in accordance with claim 1, where the amine accelerator is di-n-hexylamine.
 6. A method for making a room temperature vulcanizable composition which comprises agitating under substantially anhydrous conditions at a temperature in the range of from 0° C. to 180° C. a mixture of ingredients comprising by weight,(A) a silanol terminated polydiorganosiloxane (B) an effective amount of a condensation catalyst (C) 0.001 to 0.1 part per part of the silanol terminated polydiorganosiloxane of a cross-linking polyalkoxysilane (D) from 0 to 0.1 part per part of the silanol terminated polydiorganosiloxane of an amine accelerator (E) a stabilizing amount of a N-silyl-substituted imidazole of the formula ##STR6## where R¹ is selected from C.sub.(1-8) aliphatic radicals, substituted C.sub.(1-8) aliphatic radicals, C.sub.(6-13) aromatic hydrocarbon radicals and substituted C.sub.(6-13) aromatic hydrocarbon radicals, and R², R³ and R⁴ are selected from hydrogen, C.sub.(1-8) alkyl, C.sub.(1-8) alkenyl, and C.sub.(6-13) aryl, and a is an integer equal to 1-4 inclusive.
 7. A method for making a room temperature vulcanizable composition which comprises agitating under substantially anhydrous conditions at a temperature in the range of from 0° C. to 180° C. a mixture of ingredients comprising by weight,(A) an organopolysiloxane wherein the silicon atom at each polymer chain end is terminated with at least one alkoxy radical, (B) an effective amount of a condensation catalyst (C) 0 to 0.1 part per part of the alkoxy terminated organopolysiloxane of a cross-linking polyalkoxysilane (D) from 0 to 0.1 part per part of the alkoxy terminated organopolysiloxane of an amine accelerator (E) a stabilizing amount of a N-silyl-substituted imidazole of the formula ##STR7## where R¹ is selected from C.sub.(1-8) aliphatic radicals, substituted C.sub.(1-8) aliphatic radicals, C.sub.(6-13) aromatic hydrocarbon radicals and substituted C.sub.(6-13) aromatic hydrocarbon radicals, and R², R³ and R⁴ are selected from hydrogen, C.sub.(1-8) alkyl, C.sub.(1-8) alkenyl, and C.sub.(6-13) aryl, and a is an integer equal to 1-4 inclusive. 